1. Field of the Invention
This invention relates to Pt based nano-size particles for use in fuel cells and the like, distributed within/on high surface area, preferably electronically conductive support materials and to the making of electrodes therefrom. This invention also involves the synthesis of Pt based fuel cell (FC) catalysts in the nano-size range that are electro-catalysts for methanol, hydrogen, carbon monoxide oxidation and oxygen reduction. This invention also relates to the deposition of the fuel cell catalysts on desired supports that are preferably electronically conductive and of high surface area, as well as on/into substrates of complex structure and high internal surface area.
2. Description of the Prior Art
Direct methanol and polymer electrolyte membrane fuel cells (referred to as DMFCs and PEMFCs, respectively) preferably operate using catalysts containing Pt and in some cases one or more ad-metal and/or ad-metal oxides1. Particularly due to the high price of noble metals a good catalytic performance employing low noble metal catalysts loadings is desired. The state of the art technology involves the preparation of dispersed noble catalysts onto carbon blacks that are subsequently dispersed using suitable chemicals/solutions forming a so called “ink” that is then applied and pressed onto the FC membrane2.
Much information regarding the preparation of FC catalysts is proprietary. Reported synthesis methods of these noble metal catalysts often involve the reduction of noble metal salts using chemical reduction methods or reduction of them at high temperatures. Previous catalyst preparation methods involved the synthesis of Pt and PtRu, etc., colloids in the 2, 3-4 and larger than 5 nm range3. The synthesis methods often involve multiple steps and organic solvents such as e.g., tetrahydro furan (THF) that are volatile, toxic and in the case of THF form unstable, and violently reacting peroxides. Often complex and large organic compounds (called a stabilizer) are used to stabilize the noble metal colloids in solution through, normally chemically, and hence, strong interactions between the stabilizer and the catalyst particles. Subsequently the catalysts are applied onto the support, which is typically carbon black. The stabilizers are suggested to be removable from the noble metal catalysts by heat treatment (i.e., temperatures>200° C.)4. In some cases, the stabilizer also acts as the reducing agent for the noble metal salts or a chemical reducing agent such as sodium borohydride is employed.
Other catalyst synthesis methods involve high temperature (typically 400-600° C.) methods and H2 or synthesis gas (H2/N2 mixture) as reducing agent. Lengthy and energy-consuming ball-mill processes for the preparation of FC catalysts have also been tested5. Many of these synthesis methods are lengthy, time-consuming processes that involve toxic reactants and/or solvents that are hazardous. Furthermore, high temperatures are often involved in the synthesis of these catalysts that is likely to result in sintered, agglomerated and dehydrated forms of the catalysts. Sintering and agglomeration processes result in larger particles, thus decreasing the electro-active catalyst area per weight of the catalyst. Furthermore, dehydrated forms of the Pt based noble metal catalysts such as PtRu are less desirable than the hydrated forms, particularly for direct methanol fuel cell applications.
Also, the synthesis of single metal particles from colloidal solutions including ethyleneglycol has been described7. Wang et al.7 used water to adjust their particle size. We do not use water; in fact, the use of excessive amounts of water to make bi-metallic PtRu catalysts is likely not desirable, as it is expected to result in the formation of complicated Ru-aqueo and oxy complexes. Also, we tried to repeat their suggestion and use water to adjust the Pt particle size, however, we were not successful. Also, they state that the noble metal salts are reduced by argon, which is simply not possible.